Optical shape sensing (OSS) is an optical measurement technique for determining the position and shape of a structure in a three-dimensional space. The Optical Shape Sensing (OSS) technology is also called Fiber-optical RealShape (FORS) technology. Optical Shape Sensing can be applied in minimal invasive procedures in healthcare, wherein it is advantageous to monitor the three-dimensional shape of an elongated medical device within the body of a patient.
To this end, the OSS is based on three techniques: first, strain sensing using spectroscopy; second, distributed sensing using interferometry; and third, shape reconstruction using a special fiber geometry. State of the art OSS techniques utilize strain sensing which entails the measurement of spectral shifts. In particular, a swept laser source is applied which generates light of a chosen wavelength, wherein the wavelength can be varied within a wavelength span. Such a swept laser source is also known as tunable laser source (TLS). A light beam generated by the TLS is split into a reference beam and a device beam by a fiber splitter. The device beam is directed towards a device under test (DUT) via a circulator. The device beam is reflected within the DUT and is redirected by the circulator to a fiber coupler. The reference beam is directly guided to the fiber coupler to form an output lighting combined from the reference beam and the reflected device beam. The output light beam can be monitored by a detector in order to retrieve interference signals resulting from constructive and destructive interferences between the reference beam and the reflected device beam.
Several requirements need to be fulfilled by the TLS utilized in the current implementations of OSS. Firstly, the spectral output of the laser needs to be monochromatic so that light travelling within the medical device, in particular being reflected between different inner surfaces along the fiber encapsulated by the medical device, will still have a well-defined phase in order to give rise to a proper interference with the light that only travels along the reference path of the interferometer. In other words, the coherence length of the laser should be much larger than twice the fiber length multiplied by the refractive index of the fiber. This means that the line width should be in the MHz range or lower. The line width is the width of the spectrum while the laser is not scanning. The smaller the spectral width the better the optical frequency (in MHz) and wavelength are determined, the longer is the coherence length (speed of light/frequency width).
Secondly, the sweeping of the laser over the entire spectral range should be linear in time. The latter requirement originates from the fact that the different fiber positions will only give rise to a specific beating frequency on the detector when the frequency range swept by the TLS is linear in time. Otherwise scrambling of optical data corresponding to adjacent fiber positions will occur. However, the TLS known in the art do not sufficiently fulfill this requirement. As a consequence, it is necessary to add an additional interferometer with a fixed delay length to the system. The signal from the additional interferometer is then used to linearize all other optical signals.
Thirdly, the scanning/sweeping speed of the laser should be sufficiently large. In optical shape sensing for medical applications, the fiber is incorporated in a catheter or guide wire. These devices are manipulated by hand and therefore prone to vibrations. Nevertheless, high stability, in particular interferometric i.e. sub-wavelength stability is required. Interferometric or sub-wavelength stability means that the path difference between the reference beam and the reflected device beams during the scan to precision should be significantly smaller than the wavelength, i.e. in the nanometer regime. This can only be achieved with a short acquisition time, requiring the laser to operate at a scanning speed of 10,000 nm/s or larger. Such a large scanning speed already gives rise to interferometric signals that are not purely linearly proportional to the delay length but also exhibit additional quadratic effects in delay.
U.S. Pat. No. 7,772,541 B2 discloses a fiber optic position and/or shape sensing device including an optical fiber with either two or more single core optical fibers or a multi-core optical fiber having two or more fiber cores. U.S. Pat. No. 7,781,724 B2 discloses a fiber optic position and shape sensing device comprising an optical fiber means, which comprises at least two single core optical fibers or a multi-core optical fiber having at least two fiber cores. These fiber optic position and shape sensing devices known in the art utilize swept laser sources to generate light for performing OSS.